1. Field of the Invention
The present invention relates to a method of manufacturing an electronic part, an electronic part, and an electroless plating method. Particularly, the present invention relates to a method of manufacturing a chip-type electronic part such as a monolithic capacitor, a noise filter, or the like, and an electronic part manufactured by the manufacturing method.
2. Description of the Related Art
In the field of electronic parts, a surface of an electrode is often plated with nickel or gold for improving a bonding ability, solderability, a conductive resin bonding ability, heat resistance, corrosion resistance, etc., obtaining a highly functional electronic part having excellent reliability.
As is generally known, plating methods are roughly divided according to coating methods into electrolytic plating in which electrolysis is performed by passing a current through a plating solution containing metal ions to deposit a metal on a work piece, and electroless plating. Electroless plating includes an auto-catalytic type in which electrons produced by oxidation reaction of a reducing agent added to a plating solution are used for metal deposition reaction, and a substitutional type using substitution reaction between a base metal and metal ions in a solution.
In the auto-catalytic electroless plating, a surface of an electrode must be made catalytically active to oxidation reaction of the reducing agent. Therefore, a member to be plated is conventionally dipped in a catalyst solution containing Pd (palladium) to treat the surface of a Cu electrode so that the surface of the electrode becomes catalytically active.
However, as described above, when the member to be plated is dipped in the catalyst solution containing Pd, Pd adheres to portions other than the electrode to be made catalytically active, and thus Ni plating proceeds with Pd serving as nuclei to possibly deposit Ni on the portions other than the electrode. Furthermore, a degreasing step and an etching step are required as pre-treatments for applying a Pd catalyst, thereby complicating the manufacturing process. Therefore, this type of plating is principally performed by electrolytic plating.
On the other hand, it has recently been found that by using a boron-based compound as a reducing agent, electroless plating can be performed directly on an electrode without performing a catalytic treatment with Pd. Therefore, a technique based on this finding is proposed, in which a Nixe2x80x94B layer, a Nixe2x80x94P layer and an Au layer are successively laminated on a Cu electrode by electroless plating (Japanese Unexamined Patent Application Publication No. 10-135607).
In this conventional technique, electroless plating can be performed on a work piece without application of a Pd catalyst thereto, and thus the Nixe2x80x94B layer, the Nixe2x80x94P layer and the Au layer are successively laminated by electroless plating. Therefore, a Ni-based film and Au-based film can be formed only on the surface of the electrode without the formation of a plating film on portions other than the electrode.
However, the electroless plating by the above-described plating methods has the following problems.
(1) A multi-terminal electronic part having a plurality of terminals has difficulties in uniformly supplying electric power to the terminals, thereby causing variations in thickness of plating films due to non-uniformity in thickness of the plating films of the terminals. In this case, a necessary and minimum thickness must be secured, and thus plating conditions must be set to permit the formation of a plating film based on the minimum thickness. Therefore, the thickness of the plating film is increased as a whole, and particularly in Au plating, the cost is increased due to the high cost of a material itself. In Ni plating, a Ni plating film is likely to be separated from the electrode due to film stress.
(2) Also, in a small-size part such as a chip-type electronic part, so-called barrel plating is widely put into practical used. On the other hand, further miniaturization of electronic parts is required at present.
Namely, electronic parts are possibly caught in many holes formed in a barrel inner wall with miniaturization of electronic parts, and thus the opening diameter of the holes must be decreased. However, a decrease in the diameter of the holes worsens a flow of a plating solution.
In barrel plating, plating is performed in a barrel device in which many conductive media are charged for securing conductivity of the electrode. Therefore, the diameter of the conductive media used must be decreased to 0.8 mm or less with further miniaturization of electronic parts, and thus inexpensive media (so-called steel shots) containing relatively large conductive media of about 1 mm in diameter cannot be used. Namely, from the economical viewpoint, it is preferable to charge inexpensive steel shots having irregular shapes into the barrel device for plating. However, the opening diameter of the barrel holes must be decreased with miniaturization of electronic parts, and consequently expensive small-diameter conductive media or steel balls having a uniform shape must be used because the conductive media are possibly caught in the small-diameter holes, thereby causing an increase in the manufacturing cost.
(3) When an Au film is formed by electrolytic barrel plating, Au is deposited not only on electrodes of electronic parts but also on the conductive media. Therefore, expensive Au is excessively consumed, and particularly in Au plating for bonding, a large thickness is required to increase the manufacturing cost.
(4) Furthermore, in a low-resistance electronic part such as a varistor or the like, which comprises a ceramic material as a base material, electrons flow to the surface of the ceramic base material in electrolytic plating, and thus a plating metal is abnormally deposited on the surface of the ceramic base material. Particularly in barrel plating, a potential distribution is complicated to cause difficulties in avoiding abnormal deposition of the plating metal on the ceramic base material.
On the other hand, the electroless plating method disclosed in Japanese Unexamined Patent Application Publication No. 10-135607 is capable of plating without applying a Pd catalyst on an electrode surface, and is thus capable of forming a desired plating film only on the electrode. However, for example, when substitutional Au plating is performed directly on the surface of a Nixe2x80x94B layer, there is a fear that sufficient adhesion cannot be held between the Nixe2x80x94B layer and the Au layer, thereby causing the need to form a Nixe2x80x94P layer having good adhesion to Au on the Nixe2x80x94B layer. Therefore, the two plating films, i.e., the Nixe2x80x94B layer and the Nixe2x80x94P layer, must be formed between the electrode and the Au layer to cause the problem of complicating the production process. Furthermore, in forming the Nixe2x80x94B layer by electroless plating, an expensive reducing agent such as dimethylaminoborane ((CH3)2NHBH3; referred to as xe2x80x9cDMABxe2x80x9d hereinafter) must be used, causing the problem of increasing the manufacturing cost.
The present invention has been achieved in consideration of the above problems, and an object of the present invention is to provide an electroless plating method capable of forming a desired plating film only on a desired portion at a low cost, a method of manufacturing an electronic part, and an electronic part with excellent reliability which is manufactured by the manufacturing method at a low cost.
As described above, in barrel plating on a small-size electronic part such as a chip-type electronic part based on the electroless plating method, plating must be performed in a barrel in which small-diameter conductive media are charged, and a plating metal is also deposited on the surfaces of the conductive media to accordingly cause the need for excessive metal, thereby increasing the manufacturing cost. Furthermore, a multi-terminal electronic part causes the problem of widening a thickness distribution, and the like.
Accordingly, the inventors intensively researched in order to achieve a method of forming a plating film only on a desired portion with attention to electroless plating. As a result, it was found that even for an electrode material having no catalytic activity to oxidation reaction of a reducing agent, when metal pieces with catalytic activity to the oxidation reaction are mixed with a work piece to bring the metal pieces into contact with the work piece, the electrode of the work piece is provided with catalytic activity to permit the formation of a plating film with a desired uniform thickness only on the surface of the electrode without abnormal deposition of the metal on portions other than the electrode.
Namely, a method of manufacturing an electronic part according to the present invention comprises performing electroless plating on a work piece, which has an electrode formed on a surface thereof, by using a plating bath containing a reducing agent, wherein conductive media exhibiting catalytic activity to oxidation reaction of the reducing agent are mixed with the work piece in the plating bath containing metal ions having an electrochemically nobler potential than the oxidation-reduction potential of the reducing agent so that the plating film is formed on the electrode by electroless plating.
In auto-catalytic electroless plating, cathodic deposition reaction of the metal, and anodic oxidation reaction proceed in parallel to deposit the metal having an electrochemically nobler reversible potential than the oxidation-reduction potential of the reducing agent on the electrode as a base metal. However, in order to stationarily advance the auto-catalytic electroless plating, the deposited metal must have catalytic activity to anodic oxidation reaction of the reducing agent.
However, in the manufacturing method, the conductive media exhibiting catalytic activity to oxidation reaction of the reducing agent are mixed with the work piece to impart catalytic activity to the surface of the electrode. Therefore, electrons produced by oxidation reaction of the reducing agent are supplied to the electrode to form the desired metal only on the surface of the electrode, thereby forming a plating film thereon.
As a method of mixing the conductive media and the work piece, any desired electrolytic barrel system such as a rotating barrel system, an oscillating barrel system, an inclined barrel system, or a vibrating barrel system can be used.
Namely, the present invention is characterized in that a container containing the work piece and the conductive media is rotated, oscillated, inclined or vibrated in a plating tank filled with the plating bath to mix the work piece and the conductive media.
In conventional electroless plating, an electric current does not smoothly flow to cause disturbance in an electric current distribution unless the hole diameter of a barrel is increased to some extent, and it is thus undesirable to use small-diameter conductive media or steel shots having irregular shapes. However, in electroless plating, even when the hole diameter of the barrel device is decreased, no problem occurs, and thus the hole diameter can be decreased as much as possible. Therefore, the conductive media have a high degree of shape freedom, and thus conductive media having various shapes can be used. Namely, by using large-size conductive media preferably having an average diameter of about 1.0 mm, the conductive media can be avoided from being caught in the holes of the barrel, and even in manufacturing small-size electronic parts, expensive small-size conductive media or steel balls with a uniform shape need not be used.
Therefore, the present invention is characterized in that the conductive media preferably have an average diameter of 1.0 mm or more. The average diameter represents an average diameter of spheres, but the average diameter of various three-dimensional shapes other than spheres represents the average of maximum diameters of three-dimensional shapes.
The method of manufacturing an electronic part of the present invention is characterized in that Ni or Ni alloy pieces are preferably used as the conductive media, the work piece is mixed with the Ni pieces in the plating bath containing an Ni compound as a main component and a phosphate compound as the reducing agent to form a first plating film comprising Ni as a main component on the electrode by electroless plating, and then the work piece on which the first plating film is formed is dipped in a plating bath containing an Au compound to deposit Au on the surface of the first plating film to form a second plating film.
In the above-described manufacturing method, electrons produced by oxidation reaction of the phosphate compound are supplied onto the electrode to form the first plating film mainly composed of Ni on the electrode by oxidation-reduction reaction. When the work piece, on which the first plating film is formed, is dipped in the plating bath containing the Au compound as a main component, electrochemically less noble Ni is dissolved to release electrons, and electrochemically noble Au ions are reduced with the released electrons to form the second film (Au film) on the first plating film (Nixe2x80x94P film).
As described above, in the present invention, the surface of the electrode is made catalytically active to oxidation reaction of the reducing agent so that an auto-catalytically persistent metal is deposited to form a plating film.
Therefore, by appropriately selecting a metal having catalytic activity to oxidation reaction of any of various reducing agents, it is possible to form a plating film on a base metal having no catalytic activity.
Studies on the catalytic activity of a phosphate compound, a boron compound, a nitrogen compound, and an aldehyde compound, which are generally used as reducing agents in a wide range, have already been reported.
For example, it is reported that Au, Ni, Pd, Co and Pt are catalytically more active to oxidation reaction of sodium hypophosphite (NaH2PO2) serving as the phosphate reducing agent than Cu and Ag (Ohno et al, xe2x80x9cCatalytic Activity of Metal to Anodic Oxidation of Sodium Hypophosphite in Electroless Platingxe2x80x9d, Metal Surface Technique, Vol. 34, No. 12, 1983, pp594-599; referred to as xe2x80x9cdocument 1xe2x80x9d hereinafter).
According to document 1, by using Au, Ni, Pd, Co or Pt as the conductive media for oxidation reaction of NaH2PO2, a plating film of Ni, Co, Pt, Au, or the like can easily be formed on an electrode surface of Cu or Ag having low catalytic activity to oxidation reaction of NaH2PO2.
Namely, the method of manufacturing an electronic part of the present invention is characterized in that metal pieces composed of at least one metal selected from Au, Ni, Pd, Co, Pt and alloys thereof are used as the conductive media, and the metal pieces and the work piece are mixed in a plating bath containing a salt of a metal selected from Ni, Co, Pd and Au as a main component, and a phosphate compound as the reducing agent, to form a plating film on the electrode by electroless plating.
It is also reported that when a boron compound such as sodium tetrahydroborate (NaBH4), DMAB ((CH3)2NHBH3), or the like is used as the reducing agent, Ni, Co, Pd, Pt and Au have higher catalytic activity to oxidation reaction of the boron compound than Cu and Ag (Ohno et al, xe2x80x9cCatalytic Activity of Metal to Anodic Oxidation of Sodium Borohydride and Dimethylaminoborane in Electroless Platingxe2x80x9d, Electrochemistry, Vol. 53, No. 3, 1985, pp196-201; referred to as xe2x80x9cdocument 2xe2x80x9d hereinafter).
According to document 2, by using Ni, Co, Pd, Pt or Au as the conductive media for oxidation reaction of the boron compound, a plating film of Ni, Co or Au can be formed on an electrode surface composed of Cu or Ag having low catalytic activity to oxidation reaction of the boron compound.
Namely, the method of manufacturing an electronic part of the present invention is characterized in that metal pieces composed of at least one metal selected from Ni, Co, Pd, Pt, Au, and alloys thereof are used as the conductive media, and the metal pieces and the work piece are mixed in a plating bath containing a salt of a metal selected from Ni, Co, Pd and Au as a main component, and a boron compound as the reducing agent, to form a plating film on the electrode by electroless plating.
When the boron compound is a tetrahydroborate compound, the reversible potential of Pt is nobler than the oxidation-reduction potential of the tetrahydroborate compound, and thus the plating solution may comprise a metal selected from Ni, Co, Au, and Pt.
Namely, the plating bath preferably further contains a metal salt of a metal selected from nickel, cobalt, palladium, gold, and platinum. In other words, the plating bath preferably further contains a metal salt of a metal selected from four metals including the unselected three metals of Ni, Co, Pd and Au, and platinum.
It is further reported that when hydrazine (N2H4) as a nitrogen compound is used as the reducing agent, Co, Ni, Pt, and Pd have higher catalytic activity to oxidation reaction of N2H4 than Cu and Ag (Ohno et al, xe2x80x9cCatalytic Activity of Metal to Anodic Oxidation of Formaldehyde and Hydrazine in Electroless Platingxe2x80x9d, Electrochemistry, Vol. 53, No. 3, 1985, pp190-195; referred to as xe2x80x9cdocument 3xe2x80x9d hereinafter).
According to document 3, by using Co, Ni, Pt, or Pd as the conductive media for oxidation reaction of N2H4, a plating film of Ni, Co, Pd, Au or Pt can be formed on an electrode surface composed of Cu or Ag having low catalytic activity to oxidation reaction of N2H4.
Namely, the method of manufacturing an electronic part of the present invention is characterized in that metal pieces comprising at least one metal selected from Co, Ni, Pt, Pd, and alloys thereof are used as the conductive media, and the metal pieces and the work piece are mixed in the plating bath containing a salt of a metal selected from Ni, Co, Pd and Pt as a main component, and a nitrogen compound as the reducing agent, to form a plating film on the electrode by electroless plating.
The present invention is based on the assumption that the electrode has no or low catalytic activity to oxidation reaction of the reducing agent, and ordinary examples of the electrode material include Cu, Ag, and Agxe2x80x94Pd.
Namely, the present invention is characterized in that the electrode is a Cu electrode, a Cu alloy electrode, an Ag electrode, a Ni electrode or an Ag alloy electrode.
It is also reported in document 3 that when formaldehyde (HCHO) is used as the reducing agent, Cu, Au, and Ag have higher catalytic activity to oxidation reaction of HCHO than Ni. Therefore, by using Cu, Au or Ag as the conductive media, a plating film of Cu, Au or Ag can easily be formed on a Ni electrode having low catalytic activity to HCHO.
Namely, the method of manufacturing an electronic part of the present invention is characterized in that metal pieces composed of at least one metal selected from Cu, Au, Ag, and alloys thereof are used as the conductive media, and the metal pieces and the work piece are mixed in a plating bath containing a salt of a metal selected from Cu, Ag, and Au as a main component, and an aldehyde compound as the reducing agent, to form a plating film on the electrode (Ni electrode) by electroless plating.
An electronic part of the present invention is characterized by being manufactured by the above-descried manufacturing method.
The electronic part of the present invention can be obtained at a low cost without a surface treatment with a catalyst solution containing Pt so that only an electrode portion is plated with good uniformity in the thickness.
Particularly, a Nixe2x80x94P layer having good adhesion to Au can be formed directly on a copper electrode, thereby obtaining a high-quality electronic part having excellent reliability at a low cost.
The present invention also provides an electroless plating method comprising mixing a work piece having a portion to be plated and conductive media exhibiting catalytic activity to oxidation reaction of a reducing agent in a plating bath containing the reducing agent and metal ions having an electrochemically nobler deposition potential than the oxidation-reduction potential of the reducing agent to form a plating film on the portion to be plated based on the metal ions.
Besides the electronic part, various materials to be plated can be used, and the portion to be plated is not limited to an electrode provided on the surface of the electronic part.
In the electroless plating method, a container containing the work piece and the conductive media can be rotated, oscillated, inclined or vibrated in a plating tank filled with the plating bath to mix the work piece and the conductive media. As described above, the average diameter of the conductive media is preferably 1.0 mm or more.
In the electroless plating method of the present invention, nickel pieces or nickel alloy pieces may be mixed with the work piece in a plating bath containing a phosphate compound such as sodium hypophosphite serving as the reducing agent, and a nickel compound as a precursor of metal ions to form a first plating film mainly composed of nickel on the portion to be plated by electroless plating, and then the work piece on which the first plating film is formed may be dipped in a plating bath containing a gold compound to deposit gold on the surface of the first plating film to form a second plating film mainly composed of gold. In this case, the material of the portion to be plated is preferably copper, silver, nickel or an alloy thereof.
In the electroless plating method of the present invention, metal pieces of at least one metal selected from gold, nickel, palladium, cobalt, platinum, and alloys thereof may be used as the conductive media, and the work piece and the metal pieces may be mixed in a plating bath containing a phosphate compound serving as the reducing agent, and a metal salt of a metal selected from nickel, cobalt, palladium and gold as a precursor of metal ions, to form a plating film on the portion to be plated by electroless plating. In this case, the material of the portion to be plated is preferably copper, silver, nickel or an alloy thereof.
In the electroless plating method of the present invention, metal pieces of at least one metal selected from nickel, cobalt, palladium, platinum, gold, and alloys thereof may be used as the conductive media, and the work piece and the metal pieces may be mixed in a plating bath containing a boron compound such as sodium tetrahydroborate or DMAB serving as the reducing agent, and a metal salt of one or two metals selected from nickel, cobalt, palladium, gold and platinum as a precursor of metal ions, to form a plating film on the portion to be plated by electroless plating. In this case, the material of the portion to be plated is preferably copper, silver, nickel or an alloy thereof.
In the electroless plating method of the present invention, metal pieces of at least one metal selected from cobalt, nickel, platinum, palladium, and alloys thereof may be used as the conductive media, and the work piece and the metal pieces may be mixed in a plating bath containing a nitrogen compound such as hydrazine or the like serving as the reducing agent, and a metal salt of a metal selected from nickel, cobalt, palladium, gold and platinum as a precursor of metal ions, to form a plating film on the portion to be plated by electroless plating. In this case, the material of the portion to be plated is preferably copper, silver, nickel or an alloy thereof.
In the electroless plating method of the present invention, metal pieces of at least one metal selected from copper, gold, silver, and alloys thereof may be used as the conductive media, and the work piece and the metal pieces may be mixed in a plating bath containing an aldehyde compound such as formaldehyde serving as the reducing agent, and a metal salt of a metal selected from copper, silver and gold as a precursor of metal ions, to form a plating film on the portion to be plated by electroless plating. In this case, the material of the portion to be plated is preferably nickel.